The concept of our experiments assumes that at least one cell type required for the immune response to an antigenic determinant has receptors specific for the determinant and that the combining site of the receptor is very similar or identical to the combining site of the antibody for the determinant. We have prepared homologous mouse anti- receptor sera (anti-idiotypic) against myeloma protein with antibody activity against phosphorylcholine. The anti-receptor sera specifically blocks the antibody activity of the myeloma protein and of antibody against phosphorylcholine induced in mice or in cultures of mouse spleen cells by immunization with a vaccine of R36A strain of pneumococci. Furthermore, the anti-receptor sera also specifically suppresses the induction of the response to phosphorylcholine in vivo or in vitro. We suggest that anti-receptor antibody may be equivalent to "blocking or enhancing" antibody. With the availability of an anti-receptor antibody directed against the receptor for a single antigen, we propose to study the roles of antibody and receptors in the regulation of immune responses. To this end we will devise ways to 1) select, purify and enrich a population of cells having receptors for the single hapten specific antigen, 2) characterize the chemical nature of the antigen specific receptor, 3) compare the variable region of antibodies, myeloma proteins, and antigen receptor molecules all specific for the same single antigen determinant and 4) characterize the cell type(s) having receptors for the determinant. Finally, we will attempt to generalize our findings to determine the role of anti-receptor antibody in other immune reactions.